Cannot Connect to Live with Auto MTU

Hi CDR,

Sorry to hear about the connection troubles. Let's see what we can do.

To get a better idea of your network setup, please respond to the bold info below and we can work from there.

What Country/State/Province do you live in? 
Modem brand & model number: 
Router brand & model number: 
If Wireless, are you using a white or black external adapter, or an internal (Xbox 360S) adapter?
Things you have tried: 
Is UPnP Enabled? Yes/no/Unknown
Do you have a NAT error? If so, what is your NAT type?
Any Error Codes you encounter: 
Who is your ISP(Internet Service Provider)? 
After you run "Test Xbox LIVE connection" press “Y” for "More Info”. In the pop-up that appears, what are the following values, if shown? (If a value below does not appear for you, just leave it blank.)
W:
X:
Y:
Z:
ID:
L:
Q:
T:
D:
S:
C: 

Hey there KandakCdr! Thanks for hitting us back with that information! That's tremendously helpful in letting us get a picture of your home setup.

I see you're connecting wirelessly to your router. Is it possible for you to try a wired (via ethernet) connection from your console to your router and let us know:

1. Are you able to connect to Xbox Live with MTU Auto? If so, does your speed seem normal?
2. Are you able to connect to Xbox Live with MTU Manual? If so, does your speed seem normal?

If neither of those yield optimal results, can you also try a wired connection directly from your console to your modem (taking your router out of the equation entirely)? This will let us see if the issue lies with the router itself.

Let us know how things go and we'll be waiting to hear from you. =)